Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives an inbound RF signal via the antenna and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with one or more local oscillations to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. The filtering stage filters the baseband signal or the IF signal to attenuate unwanted out of band signals to produce a filtered signal. The data recovery stage recovers raw data from the filtered signal in accordance with the particular wireless communication standard.
The filtering stage may either be a low pass filter or a band pass filter. In either implementation, the filtering stage functions to pass signals of interest and attenuate unwanted signals. Typically, the unwanted signals are adjacent channels and noise, with the adjacent channels being the dominant source. To insure that the unwanted signals are sufficiently attenuated, the filtering stage should have a sharp roll-off at its corner frequency or frequencies, i.e., the filtering stage should have a high quality factor (Q). However, in wireless communication systems, the Q of the filtering stage cannot be indiscriminately increased to provide the desired roll-off since, under certain operating conditions, the filtering stage will become unstable.
For instance, to provide an adequate Q for filtering 1 MHz spaced adjacent channels, the filtering stage would become unstable when the signal strength of the received RF signal was slightly larger than nominal values (e.g., −40 dBm to −50 dBm). As is known, the signal strength of the received RF signal may vary from −80 dBm to 0 dBm in Bluetooth application. Thus, to accommodate this range of signal strengths, a compromise must be made between lowering the Q and limiting the range of acceptable signals strengths. In many applications, such a compromise is unacceptable.
Therefore, a need exists for a method and apparatus for controlling power levels of received RF signals such that a high Q filtering stage may adequately attenuate unwanted signals and remain stable over the desired operating range.